WN604 forgets WMM & WPA config settings

I have a WN604 which I'm using to deliver WiFi to a part of my house which is not well served by the main wireless router. It's connected via Ethernet to the main router, using the main router's IP address as the AP's gateway address, and it's operating in AP mode without DHCP. I've set the wireless security to be WPA2 only, using the same SSID and passphrase as the main wireless router.

Importantly, I've disabled the WMM extensions, since several of my devices demonstrably fail to obtain connectivity via the WN604 whenever it's enabled, and succeed only when it's disabled. (Netgear manuals state that WMM requires client support - I don't understand why it would be enabled by default when it can't be assumed that clients will support it).

So far, so good. With the config as described, the setup works well, and devices seem to pick up their connection from whichever AP has the stronger signal.

Until the WN604 reboots, for whatever reason. Then, despite me having configured the AP to disable WMM, it reboots every single time with WMM enabled. I won't necessarily know the AP has rebooted (e.g. short power cut overnight or when no-one notices), until one of my family complains that they're not getting the Internet, and I think to check the AP config. Sure enough, WMM is once again enabled. (And it's also switched itself to use both WPA and WPA2 authentication. Call me picky, but I don't want WPA - I want WPA2 only.)

I have tried saving the config as I want it, and restoring. That doesn't help. Browsing the config backup file, there doesn't seem to be any WMM enabled/disabled entry in it.

Is this a bug in the device firmware? I'm using the latest available, WN604_V3.0.2.